Chronic hypoxia induces dramatic fibro-proliferative remodeling in the pulmonary artery (PA) adventitia that is especially impressive during the neonatal period and clearly contributes to the marked increase in pulmonary vascular resistance observed. One of the key mediators of these responses is protein kinase C (PKC) zeta which regulates hypoxia-inducible genes. To study the adventitial remodeling, we used PA adventitial fibroblasts cultured from neonatal control (Fib-C) and hypoxic pulmonary hypertensive calves (Fib-H) and compared the effects of hypoxia on PKCzeta-related signaling pathways. We have observed that acute hypoxic exposure induces translocation of nuclear PKCzeta into the nuclear membrane and the cytoplasm selectively in Fib-C, but not in Fib-H. In Fib-C, PKCzeta inhibition results in sustained activation of extracellular signal-regulated kinases, ERK1/2, whereas it blocks ERK1/2 activation in Fib-H. The mechanisms by which PKCzeta can diversify its own biochemical properties in response to chronic hypoxia remain unexplored. Although PKCzeta suppresses the hypoxia-induced proliferation of Fib-C, it translates the hypoxic signal into replicative responses in Fib-H. The molecular mechanisms that confer the ability of PKCzeta to perform opposing functions, especially in proliferative responses, are unknown and will be examined here. We also found that PKCzeta regulates MAP kinase phosphatase-1 (MKP-1) expression only in Fib-C, but not in Fib-H. The biochemical pathways responsible for uncoupling of PKCzeta from the regulation of MKP-1 expression in Fib-H are not known. The aforementioned questions will be addressed by the following specific aims: 1) The development of hypoxic pulmonary hypertension leads to the appearance of fibroblasts with modified nuclear localization, activation and target specificity of PKCzeta; 2) An altered functional role of PKCzeta as a growth regulator, promotes enhanced proliferation of fibroblasts from chronically hypoxic animals; 3) Nuclear PKCzeta-mediated growth-suppressing activity of MKP-1 is absent in fibroblasts isolated from the adventitial compartment of chronically hypoxic animals. Successful completion of the proposed experiments will provide new insights into the role of PKCzeta and MKP-1 in pulmonary vascular diseases and could lead to the development of improved therapeutic strategies for the treatment of pulmonary diseases caused by chronic hypoxia.